Recently, along with enhancement of recording density of optical recording mediums such as optical discs, development of the technique of effectively using the area on the recording surface of an optical disc has been pushed forward to enhance the recording density.
The recording surface of any disc substrate has concavo-convex called lands and grooves respectively. In conventional optical discs, only lands or only grooves were used to record recording marks. However, with the need of enhancing the recording density of optical discs, a technique for recording record marks on both lands and grooves, i.e. a land/groove recording technique, has been brought into use as a format of optical discs. An example of optical discs combined with the land/groove recording as their format is DVD-RAM (Digital Versatile Disc-Random Access Memory).
However, such optical discs employing the land/groove recording suffer a difference of the thermal property exerted to the recording layer because of the difference in physical configuration between lands and grooves when recording laser beams are irradiated to lands and grooves respectively. In addition, lands and grooves form a periodical structure close to the wavelength of the laser beams. Therefore, vector diffraction of light causes uneven distribution of irradiated laser beams between lands and grooves, and it has been difficult to record or reproduce record marks from both lands and grooves with a high C/N ratio (carrier-to-noise ratio).
To match lands and grooves in property, it is contemplated, for example, to change widths of lands and grooves and thereby change the duty ratio. However, only change of the duty ratio cannot bring about ample freedom. Under the circumstances, an optical disc was proposed, in which the recording layer on grooves is formed thicker than the recording layer on lands by selecting the sputtering condition, for example (Japanese Patent Laid-Open Publication No. 2000-215511, referred to as Literature 1).
However, in case of the optical disk disclosed in Literature 1, the condition for deposition of the recording layer is changed to form the recording layer on grooves thicker than the recording layer on lands. This condition digresses the optimum condition for deposition of the recording layer and invites deterioration of signal characteristics of the recording layer.
Therefore, it has been demanded to develop a method capable of independently controlling the recording characteristics of lands and grooves.
It is therefore an object of the invention to provide an optical recording medium having a multi-layered film of a recording layer and a record-assist layer stacked adjacently on one surface of a disc substrate having formed a concavo-convex structure, and its manufacturing method, which enable freer designing while ensuring good signal characteristics in the recording layer on the concavo-convex structure and ensuring high reliability.